Recently, higher mounting densities and larger capacities of large-scale integrated (LSI: Large Scale Integration) circuits are further reducing the circuit line widths needed for semiconductor devices. By using an original image pattern (that is, a mask or a reticle, hereinafter collectively referred to as a mask) in which a circuit pattern is formed, the pattern is exposed and transferred onto a wafer by a reduction projection exposure apparatus, called a stepper or a scanner, to form a circuit on the wafer, thereby producing a semiconductor element.
Since LSI production requires a large manufacturing cost, it is crucial to improve the production yield. On the other hand, in a contemporary semiconductor device, a pattern having a line width from ten nanometers to twenty nanometers is required to be formed. At this point, a defect of the mask pattern can be cited as a large factor of degradation in the production yield. As the dimensions of an LSI pattern to be formed on a semiconductor wafer become finer, so too will the defects of the mask pattern become finer.
As fluctuations of various process conditions are absorbed by enhancing dimensional accuracy of the mask, it is necessary to detect the defect of the extremely small pattern in a mask inspection. Therefore, high accuracy is required for a mask inspection apparatus that inspects patterns of a mask.
In the mask inspection apparatus, light emitted from a light source is irradiated onto a mask through an optical system. The mask is loaded and chucked on a stage, and the illuminated light scans the mask by movement of the stage. The light transmitted through or reflected by the mask, images on a sensor through lenses of an optical system. Then, the defect inspection with respect to the mask is performed based on the optical images acquired by the sensor.
A die-to-die comparison inspection method and a die-to-database comparison inspection method are known as examples of mask inspection methods performed using the mask inspection apparatus. In the die-to-die comparison method, an optical image of a pattern and another optical image of the identical pattern at a different position are compared with each other. On the other hand, in the die-to-database comparison method, a reference image generated from design data used in mask production and an optical image of the actual pattern formed in the mask are compared with each other.
The optical image is generated by using a charge accumulation type time delay integration (TDI) sensor and a sensor amplifier that amplifies an output of the TDI sensor. In a case where the inspection is performed by the transmitted light, for example, a half-tone type phase shift mask can obtain the contrast of a light blocking film and a glass substrate to some degree. Therefore, like a chromium mask, there is adopted a method of performing defect determination by recognizing mask patterns through a light intensity signal of a sensor image that receives light by a detection optical system.
Depending on the shape of the defect, there is a case where the contrast is easily obtained by using light reflected from a surface of a mask, and there is an inspection method using a reflection inspection optical system for the purpose of a particle inspection function or the like. In addition, there is adopted a method of performing defect inspection with high detection sensitivity by easily correcting out-of-focus of transmitted illumination light by a variation in a mask thickness.
In the die-to-database comparison inspection, an increase in the amount of design data is progressing along with the miniaturization of patterns. Thus, it is difficult to generate design data or reference image data in real time.
That is, in order to perform the die-to-database comparison inspection, it is necessary to process a huge amount of data at a high speed, and it is therefore difficult to match a reference image generation with a scan speed for acquiring an optical image.
On the other hand, since an increase in the capacity of the storage is progressing, it is possible to store large amounts of data. Therefore, instead of a method of comparing a reference image with an optical image in real time, JP 2010-071893 A discloses an entire image acquisition and inspection method of acquiring an optical image of an entire inspection region and comparing the acquired optical image with a reference image in an offline manner, that is, not in real time.
In order to perform the die-to-database comparison inspection in real time, it is necessary to process a huge capacity of data at a high speed, and it is difficult to match a reference image generation with a scan speed for acquiring an optical image. However, since an increase in the capacity of the storage is progressing, storing large capacity data has become easy.
As described above, instead of a method of comparing a reference image with an optical image in real time, there is disclosed an entire image acquisition and inspection method of acquiring an optical image of an entire inspection region and comparing the acquired optical image with a reference image in an offline manner, that is, not in real time. Therefore, it is considered that the entire image acquisition and inspection method takes advantage of the fact that a sensor image, that is, an optical image is previously acquired.
The present invention has been made in view of these points and is directed to provide a mask inspection apparatus and a mask inspection method, which can generate a high-accuracy reference image by using an optical image previously acquired in an entire image acquisition and inspection method and can improve the inspection accuracy of die-to-database comparison inspection.